A conventional batteries use uniform cathode active materials. The (averaged) composition of small and large particles is the same. Uniform materials also have a similar composition in the inner and outer bulk of a single particle.
LiCoO2, charged to 4.4V or higher voltage is the superior material regarding reversible capacity, gravimetric and especially volumetric energy. Unfortunately, LiCoO2 charged to ≧4.4V shows high capacity fading, low safety, and in contact with the electrolyte reactivity (electrolyte oxidation) is observed.
Commercial rechargeable lithium batteries almost exclusively apply LiCoO2 as cathode active material. LiCoO2 delivers 137 mAh/g reversible capacity if charged to 4.2V; approx. 155 mAh/g reversible capacity if charged to 4.3V; approx. 170 mAh/g reversible capacity if charged to 4.4V; and approx. 185 mAh/g reversible capacity if charged to 4.5V. An increase of charging voltage to 4.4 or 4.5V could drastically increase the energy density of batteries compared with the standard 4.2V charging. Unfortunately, unprotected LiCoO2 cannot be cycled at >4.3V because of poor capacity retention and poor safety properties.
Coating of LiCoO2 particles has been suggested to protect the surface from unwanted reactions between electrolyte and the charged (=delithiated) LixCoO2. The coating approach is for example described by Y. J. Kim et all., J. Electrochem. Soc. 149 A1337, J. Cho et all., J. Electrochem. Soc. 149 A127, J. Cho et all., J. Electrochem. Soc. 149 A288, Z. Chen et all., J. Electrochem. Soc. 149 A1604, Z. Chen, J. Dahn, Electrochem. and solid-state letters, 5, A213 (2002), Z. Chen, J. Dahn, Electrochem. and solid-state letters, 6, A221 (2003), J. Cho et all., Electrochem. and solid-state letters, 2, 607 (1999), J. Cho and G. Kim, Electrochem. and solid-state letters, 2, 253 (1999), J. Cho et all., J. Electrochem. Soc. 148 A 1110 (2001), J. Cho et all., Electrochem. and solid-state letters, 3, 362, (2000), J. Cho et all., Electrochem. and solid-state letters, 4, A159, (2001), Z. Whang et all., J. Electrochem. Soc. 149, A466 (2002), J. Cho, Solid State Ionics, 160 (2003) 241-245.
Coating can to some degree improve certain properties like fading and safety. It is however not clear if this is caused by the coating layer. In Z. Chen, J. Dahn, Electrochem. and solid-state letters, 6, A221 (2003) as well as in Z. Chen, J. Dahn, Abs 329, 204th ECS Meeting, Orlando, it was shown that a similar treatment (wash+heat) without applying a coating layer causes the same improvement of cycling stability. The improvement however is temporary and vanishes after storage of the cathode.
Different mechanisms cause the fading of cathode active materials like LiCoO2. A first is the precipitation of reaction products of decomposed electrolyte onto the surface of LiCoO2 forming resistive surface layers. A second is the chemical decomposition of LiCoO2 in the presence of electrolyte, thereby changing the outer bulk chemically and structurally. A third is the degradation of bulk LiCoO2 occurring in the absence of electrolyte. This degradation can be a crystal structural degradation (for example transformation to spinel) or a morphological disintegration (electrochemical grinding, causing loss of electrical contact of crystallites). The first and second mechanism can be prevented or reduced by coating. The third requires a modification of the bulk.
Similar as the capacity fading, safety problems are also caused by different mechanism. First, delithiated LiCoO2 tends to oxidize electrolyte, which is a strong exothermic reaction. If the local temperature is high enough, the electrolyte oxidation becomes fast, more heat evolves and the battery might go to thermal runaway. Secondly, delithiated LiCoO2 in the bulk itself is unstable and might collapse towards denser phases, releasing modest amounts of heat. The reaction not involves electrolyte. The first mechanism can be prevented or reduced by coating. The second requires a modification of the bulk.
In most cases the coating accounted for less than 2-5% of the weight of the cathode active material. The stoichiometry of the total cathode active material is only marginally changed, coated active materials are basically uniform materials, because the composition of large and small particles is similar, and the composition of inner and outer bulk is basically the same.
The described coating approaches have not fully solved the stability problem at >4.3V. Particularly unsolved problems are one or more of:                Non complete coating of surface. For example, a wetting of the cathode active material powder with a gel or solution followed by a drying typically does not result in a completely covered surface.        Not enough adhesion between coating layer and cathode active material. During electrode processing and during cycling (change of crystallographic unit cell volume of LiCoO2 as function of state of charge) significant strain occurs. The strain causes a peal-off of the coating layers, leaving large areas unprotected. This problem is especially pronounced if the coating layer and the cathode active material do not form a solid state solution.        Chemical incapability. After coating usually a heating step is applied. During the heating the coating layer might decompose the cathode active material. For example, coating LiCoO2 with lithium manganese spinel is difficult or impossible because the spinel and LiCoO2 contacting each other decompose forming cobalt oxide and Li2MnO3.        Conduction problems. Insulators (as Al2O3, ZrO2 . . . ) are suggested for the coating layers. A particle, fully covered by an insulator, is electrochemically inactive. If the surface is fully covered, then the layer has to be extremely thin (to allow “tunneling” of electrons). It is questionable if such thin layers can be achieved and if they will prevent the electrolyte-surface reactions.        Coated layers are to thin to improve the safety.        Sharp two phase boundaries. If the LiCoO2 and the coating layer do not have a solid state solution, then lattice strains are localized at the boundary, which reduces the mechanical stability. A braking of particles during extended cycling is possible.        
Complex cathode active materials with layer structure have been disclosed. Some show a better cycling stability than LiCoO2 if cycled at >4.3V, and they also show better safety. Typical examples are layered cathode active materials being solid state solutions within the ternary system, LiMn1/2Ni1/2O2—LiNiO2—Li[Li1/3Mn2/3]O2—LiCoO2. In the following a short notation for the transition metal composition will be used, “ABC” refers to a lithium transition metal oxide with transition metal composition M=MnANiBCoC.
Some examples are:
    “110” —LiNi1/2Mn1/2O2 or Li[Lix(Mn1/2Ni1/2)1−x]O2, x≧0, |x|<<1 (Dahn et al. in Solid State Ionics 57 (1992) 311, or T. Ohzuku, Y. Makimura, 2001 ECS meeting (fall), Abstr. 167)    “442” —LiMO2 or Li[LixM1−x]O2 M=(Mn1/2Ni1/2)1−yCoy, x≧0, |x|<<1, y=0.2 (Paulsen & Ammundsen, 11th International Meeting on Lithium Batteries (IMLB 11), Cathodes II, Ilion/Pacific Lithium)    “111” —LiMn1/3Ni1/3Co1/3O2 (Makimura & Ohzuku, Proceedings of the 41st battery symposium on 2D20 and 2D21, Nagoya, Japan 2000 or N. Yabuuchi, T. Ohzuku, J. of Power sources 2003, (in print)    “118” —LiCo0.8Mn0.1Ni0.1O2 (S. Jouanneau et all., J. Electrochem. Soc. 150, A1299, 2003)    “530” —Li[Li1/9Mn5/9Ni1/3]O2, “530mod” —Li[Li1/9Mn5/9Ni1/3]O1.75 (J. Dahn, Z. Lu, U.S. patent application 2003/0108793A1, Z. Lu et all., J. Electrochem. Soc. 149 (6) A778 (2002))
Despite of some improvements these materials are not truly competitive. Remaining problems are one or more of:                High cost: “118” for example has raw materials costing similar as LiCoO2, however, compared to LiCoO2 which can be prepared by cheap routes (solid state reaction) the cost of preparation (typically involving mixed precursors like mixed hydroxides) is much higher.        Low volumetric energy density: Low cobalt materials like “110” or “442” have low Li diffusion constant. To obtain a sufficient rate performance, powders consisting of particles with smaller crystallites and some porosity of particles are required. The obtained porosity of electrodes is too high. Additionally, the crystallographic density is significantly smaller than LiCoO2 (5.05 g/cm3). 110 has a density of approx. 4.6 g/cm3, “442” has approx. 4.7 g/cm3. The same applies for “530” with a low density of 4.4 g/cm3. cathode active material (like “530”) are not stable. They transform to an oxygen and lithium deficient cathode active material at >4.5V during first charge. After discharge a different material “530mod” is achieved. “530mod ”        Side reactions: Manganese and lithium rich cathode material like “530” is oxygen deficient and not thermodynamically stable. Even if the electrochemical properties of the resulting material are excellent, the transformation involves the release of oxygen, possibly reacting with the electrolyte and forming undesired gas.        
Important for real batteries is not only the gravimetric reversible capacity (mAh/g) but also the energy density (=capacity×average voltage), here especially important is the volumetric energy density (Wh/L) of the electrodes. Essential to achieve a high volumetric energetic density of electrodes is (a) high powder density, (b) a large capacity and (c) high voltage.
LiCoO2 allows achieving powder densities of up to 3.5-4 g/cm3. This corresponds to approx. 70-80% of crystallographic density, or 20-30% porosity. Electrodes of complex layered materials or phosphates usually have a higher porosity. Additionally the crystallographic density of the complex layered materials is 5-12% lower. The crystallographic density of LiFePO4 is 30% lower. The same applies for spinel materials. This further reduces the energy density.